A comparative study of some hydroxyanthraquinones as acid-base indicators

Alizarin, Alizarin S, quinizarin and quinalizarin have been compared as acid-base indicators with Bromocresol Green and Methyl Orange as reference indicators. The chromaticity co-ordinates, complementary chromaticity co-ordinates, pK(a) values, transition pH-range, pH of maximum colour change, optimum concentration for titrations and the quality of colour change were determined. The results show all four to be good indicators with a colour change quality similar to that of Bromocresol Green.     

Sorption of cobalt on organic and inorganic intercalated clays

A Mexican montmorillonite clay was intercalated on the one hand with aluminium or zirconium polyhydroxications, and on the other with two organic compounds. Radioactive cobalt was used to study the Co²⁺ sorption curves in the original and pillared clays. It was found that pillaring in general does not favour the diffusion of cobalt between the layers specially the organic pillared clays. In equilibrium, the cobalt retention reached the highest level, around 0.7 meq/g in the Zr pillared clay.     

Substituent Effects on the Low-Lying Singlet and Triplet States of Methylene

The relative energies of the three lower-lying singlet states (here called S_a, S_b, and S_c for the sake of generality) and the lowest triplet state of CHX and CX₂ carbenes (in which X = Li, BeH, BH₂, NH₂, OH, or F) are evaluated by means of the semiempirical MNDO method as well as, for some species, by means of ab initio calculations at the 6-31G, MP3/6-31G, and MP3/6-31G* levels. Calculations for CH(CN) and C(CN)₂ are also reported. In spite of the known MNDO overestimation of the stability of the σ¹π¹ configurations of methylene, this method turns out to be satisfactory for most carbenes reported here. Emphasis is put on the appearance of the plots of the ΔH values vs. the carbene bond angles for the different states and on the seldom considered S_b states (¹B₁ for C_2v carbenes). A carbene classification is proposed on the basis of the form of these plots. For carbenes with π-acceptor substituents such as those of “type IA”, open-shell, diradical configurations are predicted for the lowest singlet states, so that no significant structural differences should be expected between their lowest singlet and triplet states. On the other hand, for carbenes with strong π-donor substituents, either “type ID” or “IID”, the closed-shell singlets appear to be the ground states, and the singlet and triplet behaviors should be much more clearly distinguishable.     

Reduction of azides to amines mediated by tin bis(1,2-benzenedithiolate)

[reaction: see text] A procedure for the conversion of azides to amines, which uses NaBH4 and catalytic amounts of tin(IV) 1,2-benzenedithiolate, is disclosed. Primary, secondary, tertiary, aromatic, and heteroaromatic azides are reduced in excellent yields under very mild conditions.     

A direct, efficient method for the preparation of n6-protected 15n-labeled adenosines

N6-Protected adenosines have been prepared from inosines by activation of the C6 position and Pd-catalyzed coupling with amides. An efficient route to [6-15NH2]-N6-benzoyladenosine and [1-15N,6-15NH2]-N6-benzoyladenosine has been achieved.     

Dissociation constants, neutralization enthalpies and reactions of 3-styryl-2-mercaptopropenoic and 3-(1-naphthyl)-2-mercaptopropenoic acids

Dissociation constants (pK(a1) and pK(a2) in water-ethanol medium for 3-styryl-2-mercaptopropenoic and 3-(1-naphthyl)-2-mercaptopropenoic acid have been determined potentiometrically, and pK(a2) for both in aqueous medium, spectrophotometrically. Neutralization enthalpies in water-ethanol medium have been determined by thermometric titration. The reactions with metal ions have been studied, and the main reactions are described. The most sensitive reactions are with titanium(IV) (pD = 7.00) and nickel(II) (pD = 6.50).     

A comparative study of the behaviour of Crystal Violet,alizarin-9-imine and quinalizarin-9-imine as indicators in anhydrous acetic acid medium

A comparative study has been made of the behaviour of alzarin-9-imine, quinalizarin-9-imine and the widely used Crystal Violet as indicators in anhydrous acetic acid medium. The protonation constants of quinalizarin-9-imine, and the properties required by a base if it is to be titratable in acetic acid medium with use of the indicators studied, the chromaticity co-ordinates, optimal concentration for titration and the quality of the colour changes have been determined. The results obtained show Crystal Violet and alizarin-9-imine to be very good indicators in acetic acid medium but alizarin-9-imine gives better end-point location. Quinalizarin-9-imine is also a good indicator but exhibits poorer colour change quality than the other two.     

Insight into the mechanism of the asymmetric addition of alkyl groups to aldehydes catalyzed by titanium-BINOLate species

The asymmetric addition of alkyl groups to aldehydes catalyzed by BINOLate-titanium complexes has become the testing grounds to evaluate the potential of new BINOL-based ligands. We have investigated the mechanism of this reaction and report our findings here. Model systems for the open form of the catalyst, (BINOLate)[Ti(O-i-Pr)(3)](2), based on mono-oxygen-alkylated BINOL ligands have been examined. Comparison of the reactivity and enantioselectivity of the mono-alkyl BINOL derivatives with those of BINOL indicate that the open form of the catalyst, (BINOLate)[Ti(O-i-Pr)(3)](2), is not active in the asymmetric addition reaction. Several BINOLate-titanium complexes have been synthesized and characterized by X-ray crystallography. These include the dinuclear (BINOLate)Ti(O-i-Pr)(2).Ti(O-i-Pr)(4), which contains a bridging naphtholate and isopropoxy group, trinuclear (BINOLate)Ti(O-i-Pr)(2).[Ti(O-i-Pr)(4)](2), and trimeric [(BINOL)Ti(O-i-Pr)(2)](3). The solid-state and solution structures reported here indicate that (BINOLate)Ti(O-i-Pr)(2) prefers to bind to titanium tetraisopropoxide rather than to itself, explaining why no nonlinear effects are observed in the catalytic reaction. Additionally, experimental evidence suggests that the BINOLate-titanium species responsible for the catalytic and stoichiometric asymmetric addition reactions are different, indicating that the proposed intermediate, (BINOLate)Ti(R)(aldehyde)(O-i-Pr), is not involved in either of these processes. Reactions were examined using different sources of the alkyl group [ZnMe(2) or MeTi(O-i-Pr)(3)]. Under similar conditions, it was found that the product ee's were the same, independent of whether ZnMe(2) or Me-Ti(O-i-Pr)(3) was used as the source of the alkyl groups. This indicates that the role of the dialkylzinc is not to add the alkyl group to the carbonyl but rather to transfer the alkyl group to titanium. On the basis of these results, we hypothesize that the intermediate in the asymmetric addition involves (BINOLate)Ti(O-i-Pr)(2)(aldehyde).MeTi(O-i-Pr)(3).     

Hydrophobic and cation exchange mechanisms in the retention of basic compounds in a polymeric column

A cation exchange retention mechanism concomitant with the well-known hydrophobic partition mechanism in a polymeric column has been observed and investigated. This exchange process is attributed to ionization of some acidic sites present in the polymer column at basic mobile phase pH values. Several drugs of different basicity have been chromatographed on a polymeric PLRP-S column with methanol-water and acetonitrile-water mobile phases. The cation exchange between the protonated basic drug and the buffer cations (Na+, K+ and BuNH4+) is observed at the pH range where the protonated drug and the ionized sites of the column coexist. This process produces a shift of the retention versus pH plot of the base to pH values lower than those expected from the pKa of the base as well as a maximum in the plot at basic pH values. These effects are more pronounced for acetonitrile-water mobile phases.     

Discrepancy between the spin distribution and the magnetic ground state for a triaminoxyl substituted triphenylphosphine oxide derivative

The magnetic interaction and spin transfer via phosphorus have been investigated for the tri-tert-butylaminoxyl para-substituted triphenylphosphine oxide. For this radical unit, the conjugation existing between the pi* orbital of the NO group and the phenyl pi orbitals leads to an efficient delocalization of the spin from the radical to the neighboring aromatic ring. This has been confirmed by using fluid solution high-resolution EPR and solid state MAS NMR spectroscopy. The spin densities located on the atoms of the molecule could be probed since (1)H, (13)C, (14)N, and (31)P are nuclei active in NMR and EPR, and lead to a precise spin distribution map for the triradical. The experimental investigations were completed by a DFT computational study. These techniques established in particular that spin density is located at the phosphorus (rho=-15x10(-3) au), that its sign is in line with the sign alternation principle and that its magnitude is in the order of that found on the aromatic C atoms of the molecule. Surprisingly, whereas the spin distribution scheme supports ferromagnetic interactions among the radical units, the magnetic behavior found for this molecule revealed a low-spin ground state characterized by an intramolecular exchange parameter of J=-7.55 cm(-1) as revealed by solid state susceptibility studies and low temperature EPR. The X-ray crystal structures solved at 293 and 30 K show the occurrence of a crystallographic transition resulting in an ordering of the molecular units at low temperature.